In digital image processing it is often useful to detect human irises in an image. This information is used, for example, to locate other features in the image such as human eyes. This information can also be used for other purposes such as finding the orientation of a human face in the image.
Methods are known in the art for detecting human eyes. For example, U.S. Pat. No. 6,072,892 discloses the use of a thresholding method to detect the position of human eyes in a digital image. In this method, a scanning window scans across the entire image using a raster scanning method. A histogram extractor extracts intensity histograms from the window as it scans across the image. Each intensity histogram is examined by a peak detector to find three peaks in the histogram representing the skin, the white of the eye, and the black of the pupil located within the iris. A histogram having the three peaks identifies a location in an image that potentially defines an eye position. Eye position is determined from among the potential locations by calculating the area under the histogram associated with each potential location and by selecting the location associated with the histogram having the largest area.
One of the problems with this approach is that the entire image must be scanned on a pixel-by-pixel basis. Thus, a search window must be positioned at each pixel and a histogram must be assembled at each pixel location. Further, the area under each histogram must be calculated and stored. It will be appreciated that this method consumes enormous amounts of computing power and reduces the rate at which images can be processed. This method can also produce a high rate of false positives.
Methods are also known to detect eyes that have abnormally high red content. Such abnormal red content is commonly associated with a photographic phenomenon known as red eye. Red eye is typically caused by a flash of light that is reflected by a pupil. As is described in commonly assigned and co-pending U.S. patent application Ser. No. 08/919,560, it is known to search for images for pixels having the high red content that is indicative of red eye. Similarly, commonly assigned U.S. Pat. No. 5,432,863 describes a user interactive method for detecting pixels in an image that have color characteristic of red eye. It will be recognized that these methods detect eyes only where red eye is present.
Thus, there is a need for a method of locating human irises in a digital image that is accurate and efficient.